Quantum physics—the laws that govern the behavior of smallest components of our universe, such as fundamental particles, atoms and molecules—is admittedly a tough subject, a complicated path of intricate mathematics and scientific theory. Those outside the field who brave the journey often find themselves in a confusing place where the classical principles they learned in school no longer apply and the new rules seem…well…a bit unbelievable. In the quantum world, things can be in two places at once? Better yet, they can be two things at once? What???
If this has been your experience, don’t worry—you’re in very good company. Respected scientists, including Albert Einstein, felt the same way, and made many attempts to prove that these strange new theories couldn’t be correct. Each attempt, however, failed, and instead reinforced the reality of quantum physics in contrast to our conventional intuition. But this is good news—the properties buried in quantum theory hold great promise for exciting, real-world applications.
So how do we make sense of these bizarre new rules? What really makes quantum physics so different, so strange, and so promising? To start, let’s take a look back to 1900 and the work of physicist Max Planck, who first drew back the curtain on the mysterious quantum world.
That year, Planck was embroiled in a nagging physics problem—how to explain the radiation of light emanating from hot objects. At the time, there were two conflicting laws, neither of which was quite right. Sandwiching visible light on the electromagnetic spectrum are infrared waves, which have longer wavelengths and a lower frequency, and ultraviolet waves, which have shorter wavelengths and a higher frequency. One law—Wien’s law—could accurately predict the experimental results of ultraviolet waves, but fell apart when it came to infrared waves. Conversely, the Rayleigh-Jeans law covered infrared waves, but didn’t work for ultraviolet. What Planck needed, then, was one law that would correctly apply to both ends of the spectrum.
For the birth of quantum physics, the details of Planck’s solution to this problem were far less important than the trick he used to arrive at it. This trick, which Planck later on called “happy guesswork,” was simple but unsettling: the radiation energy had to be chopped up into tiny packages, or particles of light. Based on everything physicists knew at the time, this claim was outrageous: light was understood as a wave, which left little space for particles of light, nowadays known as photons. So now light could be…both? While it was not his intent, Planck’s trick was the first step in a chain reaction that turned the physics world upside-down.
We now understand that it’s not just light, but all of the fundamental components of our universe that embrace this dual nature and the other properties of the quantum world. To explain, let’s take another step back, this time to our early science education, and picture electrons—the negatively charged fundamental particles that, together with the positively charged protons and neutral neutrons, make up atoms. Are you picturing them as miniature billiard balls? What about a light wave? Do you imagine it as a tiny version of what comes crashing against the shoreline?
These are convenient pictures, because they are easy to imagine. But what is your evidence that these mental pictures really describe the nature of an electron, and the nature of light? With your sensory perception, you cannot see a single electron, nor observe a light wave oscillate. And, as it turns out, neither light, nor electrons, nor atoms, nor even molecules are simply waves, or just particles.
When it comes to strange quantum properties, this dual wave-particle nature is just the tip of the iceberg. One of the most striking concepts is that of quantum entanglement. It can be illustrated like this: imagine being the proud parent of two children, Susy and Sam, who have just hit the age of disagreeing with each other all the time. They both like mac & cheese as well as pizza. Sadly, this is no longer sufficient to guarantee a drama-free dinner. As a counter strategy, you and your partner team up and question Sam and Susy simultaneously in different rooms. This way, they cannot coordinate their dissent, and you have a 50 percent chance of random agreement on the dinner choice.
Believe it or not, in the quantum world you would be doomed. In an experiment, the two parties could be photons, and the dinner question could be a measurement of their polarization. Polarization corresponds to the direction of oscillation—moving up and down or from side to side—when light behaves as a wave. Even if you separate the two parties, eliminating all communication, quantum physics allows for an invisible link between them known as entanglement. Quantum-Susy might change her answer from day to day (even pizza gets boring after a while), but every single time there is perfect anti-correlation with quantum-Sam’s answer: if one wants pizza, the other opts for mac & cheese—all the time!
This is just one example of the many bizarre properties we know to be true based on careful calculation and experimentation. But if we’re so sure, why do we witness so little of the quantum world?
Much of quantum physics happens at length scales so small that they remain hidden to us, even when using the most powerful microscopes. In addition, witnessing quantum physics at work turns out to be radically different from what you might call an “observation.” Seeing that an object is the color red is a fairly straightforward, unobtrusive process. Probing a quantum object like an electron or photon is an entirely different matter. True quantum behavior tends to be fragile, and attempting to measure it often constitutes a major but unavoidable disruption that usually prevents quantum weirdness from becoming directly visible.
However, just because we cannot see quantum physics in action doesn’t mean that is hasn’t affected our lives in a tangible, positive way. The impact of quantum physics has been enormous: not only is it the prime common factor in nearly all physics Nobel Prizes awarded in the past one-hundred years, but it has also been a crucial driving force in technological advances ranging from lasers and superconductors to medical imaging like MRIs. Indeed, imagining a world in which quantum physics had never been discovered would amount to eliminating a lot of the technology we take for granted each and every day.
The grandest vision, perhaps, is that of harnessing the power of quantum physics for a completely new kind of supercomputer. Such a quantum computer could solve tasks in a heartbeat that would currently require centuries of computation time on the fastest computers available today. Sounds intriguing? Many physicists around the world working on the hardware of such a machine would agree. (To learn more about what would make a quantum computer so powerful, check out the slideshow above.)
They would also explain, however, how daunting the challenges are in this endeavor. Overcoming the fragile nature of quantum behavior is not an easy task—one that rivals the quantum leap of faith taken by Planck and his colleagues to bring us into this new and exciting world.